1. Technical Field
The present invention relates to a fixing device and an endless belt assembly, and more particularly, to a fixing device and an endless belt assembly for use in an image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of these features, an image is formed by attracting developer or toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium with heat and pressure.
In general, a fixing device employed in electrophotographic image formation includes a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip. As a recording medium bearing a toner image thereupon enters the fixing nip, heat from the fuser member causes the toner particles to fuse and melt, while pressure between the fuser and pressure members causes the molten toner to set onto the recording medium.
Various methods have been proposed to provide a fast, reliable fixing process that can process a toner image with short warm-up time and first-print time without causing image defects even at high processing speeds.
For example, there is known a belt-based fixing device that employs an endless flexible belt looped into a generally cylindrical configuration extending in an axial direction thereof for rotation in a rotational, circumferential direction thereof. In this fixing device, a stationary fuser pad is disposed inside the loop of the belt, with a pressure roller disposed parallel to the belt to press against the fuser pad via the belt to form a fixing nip therebetween. For reinforcing the fuser pad against nip pressure, also provided is a generally flat, reinforcing plate having its narrow face in contact with the fuser pad inside the loop of the belt.
According to this method, the fuser belt is equipped with a tubular holder of thermally conductive metal, or heat pipe, disposed inside the loop of the fuser belt for heating the fuser belt through conduction. A heater is disposed inside the heat pipe, from which heat is imparted to the entire circumference of the fuser belt looped around the heat pipe. The heat pipe has a longitudinal side slot defined on one side thereof, within which the fuser pad is accommodated. Provision of the slotted heat pipe thus enables the fuser pad to maintain its proper operational position while subjected to external forces during operation.
Although the fixing device depicted above is generally successful, another, more simplified configuration has been proposed, in which the fuser assembly is constructed without using the heat pipe, so that the fuser belt is directly heated with a heater disposed adjacent to the fuse belt. Such arrangement would work to increase efficiency in heating the fuser belt and to reduce overall size and cost of the fuser assembly. However, simply removing the heat pipe from the fuser assembly is not practical, since absence of the longitudinally slotted heat pipe inside the belt loop translates into absence of a solid, sturdy retaining structure for retaining the fuser pad in position.
Consider a configuration in which the fuser pad has substantially no retaining structure, while provided with only a single contact portion to contact the reinforcing member. In general, such a contact portion is dimensioned substantially narrower than the width of the pad in the conveyance direction, or otherwise, is offset from the center of the pad in the conveyance direction. In such cases, without any retaining structure, the fuser pad is susceptible to displacement from its proper operational position where pressure from the pressure roller forces the fuser pad to tilt or pivot about the contact portion, resulting in dimensional variations in the fixing nip and concomitant failures, such as defective fixing performance and faulty conveyance of recording media through the fixing nip.